This present invention relates to the art of electroplating and more particularly to a method for producing electrodeposits of a thin layer of copper particularly useful for material in printed circuits.
This invention is particularly applicable to the formation of ultra-thin electrodeposited foils of copper on an aluminum carrier and will be primarily described in that context, but it is not intended in any way to be limited to foils of copper on an aluminum carrier. These foils may have a thickness on the order of 5 to 18.0 microns and are typically deposited on an aluminum sheet of 50 to 75 microns. For the purposes of this disclosure, the term aluminum is intended to embrace the commercially pure metal as well as alloys of the metal which are predominantly aluminum.
A well-known technique for manufacturing printed circuit boards involves depositing copper on a temporary carrier such as a sheet of aluminum, applying the exposed surface of the copper to a printed circuit board, such as an epoxy resin impregnated fiber glass mat or substrate, bonding the copper surface to the epoxy resin through the use of heat and pressure, and then removing the temporary carrier. In order for this laminate of thin copper foil on the resinous substrate to yield a quality printed circuit board, among other properties the thin foil must be highly pore-free and securely bondable to the substrate. In the prior art, as first disclosed in U.S. Pat. No. 3,293,109 and later adapted in U.S. Pat. No. 3,990,926, a two or more step electrodeposition of copper had been necessary to produce a foil highly pore-free and securely bondable to the resinous substrate. Typically, this involves a first copper electroplate to build up a thickness of up to about 50 microinches to ensure a uniform copper substrate base and then at least one more bath and/or a different current density to provide the greater thickness buildup as needed and to nodularize the outer copper surface for increasing the bond strength between the foil and the substrate to which it is bonded or laminated. That is, a multi-step process of electroplating copper has been needed to yield a highly pore-free foil which is securely bondable to the resinous substrate or, in other terms has a high "peel strength." "Peel strength" is a conventionally used term to refer to the strength of the bond between the foil and the resinous substrate. Peel strength in excess of about 7 lbs./in., according to the standardized measuring method ASTM D/1867 is generally deemed necessary to satisfy printed circuit requirements.
The multi-step process although capable of producing pore-free foil with a nodularized outer surface does have the drawback of requiring close control and regulation between the steps. Not only does each step need careful monitoring but also process variables of each step such as bath composition, current density in the bath, temperature, etc., must be carefully coordinated with those of each other step. For example, if a two-step process is used in which the bath composition is changed in the second step, close coordination is needed between bath composition and other variables in the first step with the new bath composition of the second step. These control and coordination requirements do not yield a simple process. Even with careful control of this multi-step process, its complexity often gives rise to reliability problems. Additionally, the multiplicity of steps would give rise to the need for more space and equipment and corresponding expense associated with them.
Accordingly, it is an object of the present invention to provide an improved process of electroplating foil.
Another object of the present invention is to provide a one-step copper electroplating process which yields a uniform, virtually pore-free copper ultra-thin foil with a nodularized surface for strong adherence to an epoxy resin impregnated fiber glass circuit board.
Yet another object of the present invention is to provide a bath which increases initial copper nucleation and provides a nodularized outer surface.
Yet another object of the present invention is to provide an improved process for electrodeposition of copper on a carrier.
Yet still another object of the present invention is to provide an improved process for electroplating pore-free copper on an aluminum carrier with a nodularized surface for strong adherence to an epoxy resin impregnated fiber glass circuit board.
Other objects and advantages of the present invention will become apparent from the following detailed description thereof, which includes the best mode contemplated for practicing the invention.